1. Field of the Invention
The invention relates to methods for manufacturing optical fiber ribbon and, more specifically, methods for manufacturing optical fiber ribbon containing optical fibers having coatings applied when the fibers are first drawn, the coatings being colored by the incorporation of a coloring agent, the methods advantageously avoiding a separate inking step and avoiding waste.
2. Description of the Related Prior Art
In conventional optical fiber ribbon, coated optical fibers usually in counts of four, eight or twelve fibers, are arranged in parallel and bundled together by a radiation-curable matrix material. Each fiber contains a primary coating and a secondary coating. The primary coating, which is applied directly onto the glass, is formulated to give a coating of low modulus and T.sub.g (glass transition temperature) in order to buffer the fiber from stresses that induce microbending of the fiber. The secondary coating is designed to have a higher modulus and T.sub.g to provide solvent and abrasion resistance.
The first step in the production of optical fiber ribbon includes the selection of appropriate optical fibers taking into consideration any relevant criteria and especially the coated diameter of the fibers. The coated fibers are then further coated, usually off-line, with an ink layer, typically 1 to 5 microns in thickness. That is, the fibers having the primary and secondary coatings thereon are removed from the spool and coated with an ink layer which is usually a radiation-curable coating composition containing a pigment system for color. The ink coating layer is then cured, and the,optical fiber having the primary, secondary and ink coating layers thereon is placed on a spool. The desired number of fibers coated in this manner are then placed on a so-called "payoff tree" which is an apparatus including one or more wheels that feed out the fibers in a manner whereby the tension in each fiber is controlled using a system of pulleys.
Following payoff from the spools, the coated fibers in the desired number are arranged in a flat plane, in a manner whereby they are just touching. The fibers are then encapsulated in a radiation-curable matrix material by passing the fibers through a first slit-shaped matrix die after which a liquid matrix material, usually a urethane acrylate material, is applied to the fibers. After passing through a second slit-shaped matrix die to remove excess matrix material, the fibers having the liquid matrix material applied thereto are subjected to actinic-radiation to cure the matrix material. The optical fiber ribbons are then placed on a spool, usually in 25 km lengths.
The application of the ink layer in a separate inking step is disadvantageous because it is costly. Additionally, in the conventional manner of applying the ink coating, lumps in the ink coating sometimes occur which causes variations in the thickness of the ink coating and breakage of the fiber. Also, matching the overall fiber diameters for a set of 4 to 12 fibers with different colored ink coatings is often difficult because of variations that occur in the thickness of the ink coating.